The present invention relates to a light modulation device in which a ferroelectric liquid crystal composition is used and a liquid crystal light modulation apparatus and in particular to a liquid crystal light modulation device and apparatus suitable for display devices which electrically control optical properties.
The prior art discloses a device which is imparted with contrast by a guest-host mode involving incorporation of a dye and which uses a liquid crystal material which directly changes from nematic N* phase to smectic Sc* phase and has a large tilt and has been proposed as a ferroelectric liquid crystal display device having a memory property with a cell large in cell thickness (which refers to the thickness of a liquid crystal layer sandwiched between two substrates hereinafter) which is easily mass-produced. (Japanese Patent Kokai No. 62-27721).
The above-mentioned prior art has problems in display quality, and it is especially difficult to increase contrast ratio. The prior art employs the guest-host mode which incorporates a dye as a guest as a means to impart contrast owing to the large tilt of the liquid crystal material. As methods for increasing contrast ratio in this guest-host mode, there have been known, for example, a method of increasing the concentration of dye to be incorporated and a method of increasing cell thickness. However, these methods still suffer from several problems. In the former method, the dye concentration cannot be increased very much because of deterioration of the liquid crystal. In the latter method, increase of driving voltage or reduction of response time are problems. For these reasons, according to the above guest-host mode, it has been difficult to obtain a contrast of, for example, higher than 10.
In addition to the above guest-host mode, a birefringence mode which uses two polarization plates has been known for imparting high contrast to liquid crystal devices. According to this method, a contrast ratio of, for example, 10 or higher which is higher than that obtained by the guest-host mode can be easily obtained by using a liquid crystal material of small tilt (theoretically 22.5.degree. as an optimum value).
However, according to the conventional birefringence mode, cell thickness must be decreased to about 1-.mu.m in order to make display of high contrast ratio with black and white colors. In this case there is the problem that incorporation of dust may cause short-circuiting between upper and lower electrodes and production thereof is difficult. Furthermore, in the case of such a thin cell, the presence of even a slight unevenness in gap brings about unevenness in density and color so that it is very difficult to obtain a uniform display.
On the other hand, in order to reduce unevenness in color in the birefringence mode, a method has been proposed according to which a structure of a ferroelectric liquid crystal molecule twisted in the direction of cell thickness is stabilized by twisting the rubbing direction of upper and lower substrates and a bright state is displayed by a twisted structure with no electrical field applied and a dark state is displayed by uniform state under application of an electrical field. (Japanese Patent Kokai No. 62-50735).
However, this method cannot afford memory and cannot make display using matrix electrodes.
As explained above, according to the conventional methods, it has been impossible to develop a stable memory and to allow a high contrast ratio and high contrast ratio with black and white colors to be compatible with each other with a thick cell (3 .mu.m or more) which can be easily produced and can provide a uniform display.